High-performance liquid chromatography (HPLC) is used to separate liquid samples into their constituent parts by means of a chromatography column (known as a column in the following text). In this case, the separation performance of the column depends inter alia on the length thereof and on the particle size of the packing material. For separation which is as good as possible, columns having a sufficient length and a small particle size are required. Such columns have high flow resistance and therefore require considerably higher pressures than conventional columns for operation.
Furthermore, sufficiently rapid separation is desired in order to allow a high sample throughput. This requires a high flowing speed in the column, with the result that the counter-pressure of the column likewise increases.
For these reasons, modern, efficient HPLC installations operate with increasingly high pressures. While pressures under 100 bar were usual in the early stages of HPLC, current HPLC pumps can sometimes deliver pressures above 1000 bar. This trend is continuing and is causing a requirement for HPLC pumps which can deliver pressures considerably higher than 1000 bar.
A basic requirement of pumps for HPLC is that the flow rate, called flow in the following text, has to be delivered as far as possible without pulsing and in a reproducible manner. In the case of gradient pumps, which are capable of mixing two or more different liquid media (also known as solvents in the following text) in a ratio which is settable in a defined manner, the mixing ratio also has to be maintained in a precisely defined manner and may not have any undesired fluctuations.